At idle speed conditions, engine combustion may have substantial variation. The variation in combustion may be due to various factors including differences in fueling, charge preparation, charge distribution, and exhaust residuals between cylinders. The variation in combustion leads to variation in cylinder pressure (e.g., cylinder IMEP) as well as cylinder torque output. The torque variations may then be transmitted to the engine mounting system leading to vibration transmission and related NVH issues. At certain frequencies, the vibration may be objectionable to the vehicle operator.
One approach to address the engine idling cylinder torque variation is shown by Nakasaka in US 2007/0163547. Therein, variation in intake air amount between cylinders is determined and a variable valve device is adjusted accordingly. Specifically, an operating angle and lift amount of the variable valve device is adjusted for each cylinder until the variations are within a permissible range.
However the inventors herein have identified a potential issue with such an approach. As an example, in engines configured with a common actuator for actuating the valves of multiple cylinders (e.g., cam based valve actuators), a change in position of the common actuator will change the valve timing of all cylinders coupled to that actuator. In addition, the same change (amount, degree, and directionality) will be effected on each cylinder. However, an actuator position that improves torque variations in a first cylinder that is coupled to the common actuator may aggravate torque variations in one or more other cylinders coupled to the actuator. Consequently, even with the valve timing adjustment, torque variations and related NVH issues may persist. Overall, engine performance may be degraded.
Thus, in one example, some of the above issues may be at least partly addressed by a method for adjusting valve timing of an engine. The method may comprise, operating intake and/or exhaust valves of two or more cylinders via a camshaft, and adjusting the camshaft during engine idle conditions for each combustion event of the two or more cylinders. In this way, a common actuator may be adjusted to compensate for cylinder-to-cylinder torque variations.
In one example, each of a first and a second cylinder on a common engine bank may be coupled to a common camshaft. The first and second cylinders may have a torque variation between them based on cylinder-to-cylinder imbalances in exhaust residuals, intake air charge, fueling, dilution, etc., between cylinders. A controller may estimate the torque variation and accordingly determine a first camshaft adjustment including a first camshaft position for when the first cylinder fires, and a second, different camshaft position for when the second cylinder fires in any given engine cycle. The camshaft adjustments may enable the torque variations between the two cylinders to be reduced. The controller may further determine camshaft adjustment limits (e.g., physical limits beyond which a position of the camshaft cannot be further adjusted) based on the current engine speed as well as the firing order of the two cylinders displaying torque variation. If the desired first camshaft adjustment is within the determined limit, then during engine idling, the controller may shift the camshaft to the first and second positions during the firing of the first and second cylinders, respectively. Herein, the first and second positions may be sufficiently separated so that the camshaft can switch between the positions at the appropriate combustion events. In this way, the torque variation may be addressed using only cam adjustments and while maintaining spark timing at MBT.
However, if the desired camshaft adjustment is outside the determined limit, then it may not be physically possible for the camshaft to switch between the positions in the allotted time. Thus, to address the torque variations, during engine idling, the controller may perform a second, different camshaft adjustment wherein the camshaft is shifted to a third position for when the first cylinder fires and a fourth, different position for when the second cylinder fires. Herein, the third and fourth positions may have a smaller separation and may not, by themselves, be able to address the torque variation. Thus, in addition to the camshaft adjustment, spark timing may be adjusted (e.g., retarded) to compensate for remaining torque imbalance of the firing cylinder.
In this way, a common actuator may be used to vary the valve timing of two or more cylinders and address cylinder-to-cylinder torque variations. By addressing the torque imbalance using camshaft adjustments, an amount of spark retard required to address the torque imbalance may be reduced, thereby improving fuel economy. By reducing torque variations arising during engine idling conditions, NVH issues may be reduced and engine performance may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.